1. Field of the Invention
The present invention relates to a surface mount crystal oscillator, and more particularly, to a surface mount crystal oscillator which lends itself to improving the productivity.
2. Description of the Related Arts
A surface mount crystal oscillator has a quartz crystal blank and an IC chip that has integrated therein an oscillation circuit using the crystal blank, both of which are contained in a package of a surface mount type. Such crystal oscillators are built in a variety of portable electronic devices in particular as reference sources for frequency and time because of their small sizes and light weights. The crystal oscillators may be classified, according to how the crystal blank and IC chip are contained therein, into a bonding type crystal oscillator which has an IC chip and a crystal oscillator contained in separate packages which are then bonded for integration, and a one-chamber type crystal oscillator which contains an IC chip and a crystal oscillator in a single recess formed in a package. An example of the bonding type crystal oscillator is disclosed in US 2004/0085147A, while an example of the one-chamber type crystal oscillator is disclosed in U.S. Pat. No. 6,720,837.
FIG. 1 illustrates a conventional bonding type surface mount crystal oscillator. Mounting substrate 2 having a recess in which IC chip 1 is contained is bonded to a bottom surface of crystal unit 3 in which crystal blank 8 is hermetically sealed. Mounting substrate 2 is formed of laminated ceramics such that the recess can be formed in one main surface thereof, and has a substantially rectangular outer planar shape. At four corners of one main surface of mounting substrate 2, that is, the four corners of an open end surface surrounding the recess, bonding terminals 5 are formed for electric and mechanical connection with crystal unit 3. Also, at four corners of the other main surface of mounting substrate 2, mounting terminals 4 are formed for use in surface-mounting the crystal oscillator on a wiring board.
IC chip 1 has electronic circuits integrated on a semiconductor substrate, where the electronic circuits include an oscillation circuit which uses crystal unit 3, and a temperature compensation mechanism for compensating crystal unit 3 for frequency-temperature characteristics. The oscillation circuit and temperature compensation mechanism are formed on one main surface of the semiconductor substrate by a general semiconductor device fabrication process. Accordingly, a circuit forming surface will herein refer to one of the two main surfaces of IC chip 1 on which the oscillation circuit and temperature compensation mechanism are formed. A plurality of IC terminals are also formed on the circuit forming surface for connecting IC chip 1 to external circuits. The IC terminals include a power supply terminal, a ground terminal, an oscillation output terminal, a pair of connection terminals for connection with the crystal blank, and the like.
Circuit terminals are provided on the bottom surface of the recess in mounting substrate 2 in correspondence to the IC terminals. The circuit terminals corresponding to the power supply terminal, ground terminal, and oscillation output terminal on the IC chip are electrically connected to mounting terminals 4 through conductive paths, not show. Circuit terminals corresponding to a pair of connection terminal of IC chip 2 are electrically connected to bonding terminals 5 through conductive paths, not shown. IC chip 1 is secured to the bottom surface of mounting substrate 2 by electrically and mechanically connecting the IC terminals to the circuit terminals through ultrasonic thermo-compression bonding using bumps 6 such that the circuit forming surface faces the bottom surface of the recess in mounting substrate 2. Then, a protection resin layer is provided within the recess of mounting substrate 2 as a so-called under-fill to fill in a space between the recess and circuit forming surface of IC chip 1 for purposes of protecting the circuit forming surface. Since bonding type crystal oscillator has a crevice which intervenes between mounting substrate 2 and crystal unit 3, external air tends to introduce into the recess of mounting surface 2, so that the under-fill is required to protect the circuit forming surface.
Crystal unit 3 in turn comprises crystal blank 8 contained in package body 7 which has a recess and is formed of laminated ceramics, and metal cover 9 bonded to an open end surface surrounding the recess to hermetically seal crystal blank 8 within the recess. In this event, metal cover 9 is bonded, through seam welding or beam welding, to a metal thick film or metal ring 14 which is disposed on the open end such that it surrounds the recess. At four corners on the outer bottom surface of package body 7, external terminals 10 are disposed in correspondence to bonding terminals 5 on mounting substrate 2. A pair of crystal holding terminals 12 are disposed on the bottom surface of the recess in package body 7 for holding crystal blank 8.
As illustrated in FIG. 2, crystal blank 8, which comprises, for example, a substantially rectangular AT-cut quartz crystal blank, is provided with excitation electrodes 5a on both main surfaces, respectively. From these excitation electrodes 5a, lead-out electrodes 11b are extended toward both ends of one side of crystal blank 8, respectively. Crystal blank 8 is secured to crystal holding terminals 12 with conductive adhesive 13 or the like at both ends of the one side thereof to which lead-out electrodes 11b are extended, thereby electrically and mechanically connecting crystal blank 8 to crystal holding terminals 12 to hold crystal blank 8 within the recess of package body 7.
In package body 7, a pair of crystal holding terminals 12 are electrically connected to a pair of external terminals 10, which are positioned on one diagonal on the outer bottom surface of package body 7, through conductive paths, not shown. External terminals 10 positioned on the other diagonal on the outer bottom surface of package body 7 are electrically connected to metal cover 9 through via holes or the like extended through package body 7.
Then, bonding terminals 5 of mounting substrate 2 are connected to external terminals 10 of crystal unit 3 using soldering or the like to connect mounting substrate 2 with crystal unit 3, thereby completing a crystal oscillator. In this event, crystal holding terminal 12 in crystal unit 3 are electrically connected to the IC terminals through external terminals 10, bonding terminal 5, and circuit terminals, causing crystal blank 8 to electrically connect to the oscillation circuit within IC chip 1.
The bonding type crystal oscillator is fabricated by independently forming mounting substrate 2 mounted with IC chip 1 and crystal unit 3, and then bonding both components. As such, if a defect is found, for example, in the crystal unit itself, the defective crystal unit can be discarded before it is bonded to mounting substrate 2, to avoid consuming expensive IC chips for nothing. Consequently, the bonding type crystal oscillator lends itself to improving the productivity.
FIG. 3 illustrates a conventional one-chamber type surface mount crystal oscillator IC chip 1 and crystal blank 8 are contained within a single recess of package body 7, and metal cover 9 is bonded to an open end surface surrounding the recess to hermetically seal IC chip 1 and crystal blank 8 within the recess. IC chip 1 and crystal blank 8 used herein are similar to those of the aforementioned bonding type one. Package body 7 is formed of laminated ceramics and has a substantially rectangular outer shape, and has the recess formed in one main surface thereof, in a manner similar to the foregoing, but differs from the bonding type one in that a step is formed on an inner wall of the recess.
Circuit terminals are disposed on the inner bottom surface of the recess in package body 7, and IC terminals of IC chip 1 are bonded to the circuit terminals through ultrasonic thermo-compression bonding using bumps 6, thereby securing IC chip 1 to the inner bottom surface of the recess. A pair of crystal holding terminals 12 are disposed on the top surface of the step on the inner wall of the recess, and crystal blank 8 is secured to crystal holding terminals 12 with conductive adhesive 13. Consequently, crystal blank 8 is positioned and held above IC chip 1 in the recess. Mounting terminals 4 are disposed at four corners on the outer bottom surface of package body 7, and mounting terminals 4 are electrically connected to the circuit terminals in a manner similar to the bonding type crystal oscillator described above. Crystal holding terminals 12 are also electrically connected to the circuit terminals through conductive paths, not shown.
In such a one-chamber type crystal oscillator, since IC chip 1 is also hermetically sealed within the recess of package body 7, no under-fill is provided for protecting the circuit forming surface of IC chip 1. Accordingly, the one-chamber type crystal oscillator has such advantages as the ability to eliminate detrimental effects due to thermal shrinkage after the under-fill is cured.
As described above, either the bonding type surface mount crystal oscillator or the one-chamber type surface mount crystal oscillator have their respective advantages, but stated another way, the bonding type one has a problem caused by the under-fill, while the one-chamber type has a problem of a lower productivity.
When the under-fill is provided for protecting the circuit forming surface of IC chip 1, the under-fill is formed by injecting a liquid protection resin, and thermally curing the protection resin. However, such under-fill exhibits large hygroscopic effects after the thermal curing, so that moisture within the under-fill is thermally expanded due to heating caused by, for example, reflow soldering for mounting the crystal oscillator on the wiring board, resulting in cracks and the like on an interface on which the bumps are bonded to the circuit terminals, and on an interface on which the bumps are bonded to the IC terminals. Thus, the bonding type is susceptible to adverse effects such as cracks caused by the under-fill. On the other hand, the one-chamber type crystal oscillator is fabricated by first securing IC chip 1 to the inner bottom surface of the recess, and then securing crystal blank 8, so that if a defect is found after crystal blank 8 has been secured, expensive IC chip 8 must be also discarded together, resulting in a lower productivity.